Manipulation of metallic particles in inverted optical tweezers

Summary form only given. A dielectric particle falling in a region of electric field gradient experiences a force towards the point of highest field. A tightly focused laser beam can create extremely high field gradients in the region of the focus. For micron-sized transparent objects suspended in fluid, the resulting gradient force may overcome the forces due to light scattering and gravity, giving a three-dimensional optical trap. These so called optical tweezers were invented over a decade ago, and have now found widespread use for biological purposes. Until recently, optical tweezers have all used a fundamental Gaussian mode, with an on-axis intensify maximum, as the trapping laser beam. Although used almost exclusively with transparent particles, metal particles of the order of 50 nm in diameter have also been trapped using a single optical beam. This is possible because at that size the Rayleigh scattering is similar for both transparent and metallic particles. Larger metal particles have also been trapped in two dimensions by using a rapidly scanned laser beam to encircle them. To the best of our knowledge, we believe that it has not previously been possible to trap micron-sized metallic particles in three dimensions.